Slices off the passed proportion of items from both ends of the passed
array (i.e., with proportiontocut = 0.1, slices leftmost 10% and
rightmost 10% of scores). You must pre-sort the array if you want
‘proper’ trimming. Slices off less if proportion results in a
non-integer slice index (i.e., conservatively slices off
proportiontocut).